1. Field of the Invention
The present invention generally relates to electrical transformer inrush current limiting, and more particularly to systems and methods for polyphase alternating current transformer inrush current limiting.
2. Related Art
Aircraft typically provide three phase wild frequency alternating current (AC) power generation capability for use by electrical systems onboard the aircraft. An electrical system typically includes a power transformer which converts the wild frequency three phase AC power provided by the aircraft into either direct current (DC) electrical power, constant frequency AC electrical power, or wild frequency AC electrical power having a different voltage. For example, the power transformer may convert the wild frequency AC power into DC constant voltage power which is then supplied to one or more electrical components via a DC bus. The power transformer may provide a substantial amount of electrical current, for example more than 1, 2, 5, 10, 20, 50, or 100 amps of current, to the one or more electrical components onboard the aircraft.
When an electrical component which is electrically coupled with the power transformer to receive electrical current therefrom is first powered on, an “inrush” of current typically flows into the power transformer due to a large capacitance of the electrical component and/or power bus to which the power transformer is coupled. This inrush current may exceed power quality requirements of the aircraft electrical power system.
An aircraft electrical power system typically has momentary power interruptions which vary from approximately 20 to 200 milliseconds (ms) in duration. Standard inrush suppression techniques which use negative temperature coefficient (NTC) thermistors are problematic in dealing with these momentary power interruptions. The NTC, which is initially open and in a state of high resistance, gradually closes as current through the NTC causes heating and a decrease in resistance. Then, after an inrush protection circuit utilizing an NTC has been utilized, the circuit typically must be maintained in an inoperative state for at least approximately one second to cool down before the inrush protection circuit can be utilized again. Consequently, the inrush protection circuit utilizing an NTC cannot provide inrush protection with momentary power interruptions which are spaced closer together than this cool down time.
In addition, the inrush protection circuit utilizing the NTC is typically continuously in the current path of the electrical components onboard the aircraft. Therefore, the inrush protection circuit must be designed to be rated for the entire current load of the electrical components which receive power through the inrush protection circuit, even after the circuit no longer provides the inrush current protection function. Therefore, the inrush current protection circuit utilizing the NTC adds additional heat dissipation, size, and weight to the aircraft electrical system.